Ultra-broad bandwidth laser glasses for short-pulse and high peak power lasers

ABSTRACT

The invention relates to glasses for use in solid laser applications, particularly short-pulsed, high peak power laser applications. In particular, the invention relates to a method for broadening the emission bandwidth of rare earth ions used as lasing ions in solid laser glass mediums, especially phosphate-based glass compositions, using Nd and Yb as co-dopants. The invention further relates to a laser system using a Nd-doped and Yb-doped phosphate laser glass, and a method of generating a laser beam pulse using such a laser system.

SUMMARY OF THE INVENTION

The invention relates to glasses for use in solid laser applications,particularly short-pulsed, high peak power laser applications. Inparticular, the invention relates to a method for broadening theemission bandwidth of rare earth ions used as lasing ions in solid laserglass mediums, especially phosphate-based glass compositions.

In high power and short-pulse laser applications, such as the presentpetawatt laser systems and ultra-short pulsed lasers (lasers producinglight pulses with a duration of, for example, around a femtosecond) aswell as the future exawatt lasers systems, it is desirable that thesolid-sate laser medium have a broad emission bandwidth. See, forexample, the Hercules laser described in Laser Focus World, April 2008,pp. 19-20, which uses Ti doped sapphire crystals.

Titanium-sapphire [Ti:Sapphire, Ti:Al₂O₃] crystals have a broad emissionbandwidth as well as high laser cross sections over a broad region ofemission. These properties, in combination with the excellent thermal,physical and optical properties of the sapphire crystal, make this thegain material of choice for active solid-state ultra-short pulsedlasers. However, the short fluorescence lifetime necessitates the needfor pumping Ti:Sapphire with other lasers (Ti doped sapphire short pulselasers are pumped by glass lasers which in turn are pumped byflashlamps). This adds to the overall architecture of lasers for scalingup to exawatt peak powers. Moreover, being a crystalline material,generating large apertures of this material with the optical qualitiesrequired has been challenging and expensive.

Rare earth doped glasses can also be used in laser architecturesdesigned to produce short-pulses. There are several advantages to usingdoped glasses over crystals. These include lower costs, and higheravailable energies since glasses can be manufactured in large sizes withhigh optical quality, while Ti doped sapphire is limited in size. Muchsimpler laser designs are possible with the glass approach, since laserglasses can be directly pumped by flashlamps. So, unlike lasers usingTi:Sapphire crystals, the glass approach does not require one to firstbuild pump lasers.

Laser glasses are produced by doping host glass systems with rare earthelements that have the ability to laser, such as neodymium andytterbium. The lasing ability of these rare earth doped laser glassesresults from the light amplification achieved by stimulated emission ofthe excited rare earth element ions within the glass.

Glasses have a proven track record as a host matrix suitable forrare-earth ions that provide the large apertures necessary for highaverage power laser systems. This is especially true for phosphateglasses which can be manufactured in large quantities and can beplatinum particle free when manufactured under the right processingconditions.

Phosphate laser glasses are well known for use as a host matrix for highaverage power and high peak energy laser systems. See, for example, U.S.Pat. No. 5,526,369 (Hayden et al.) which discloses Nd-doped phosphatelaser glasses. But, in this case, the laser glass is designed to have anarrow emission bandwidth (less than 26 nm) to improve extractionefficiency. In this typical type of laser, the emission of the laser isnarrow compared to the emission bandwidth, and thus, the emitted lightat wavelengths outside of the laser bandwidth at which the laseroperates is effectively wasted. For this reason, narrow emissionbandwidths were desirable.

Other prior work using phosphate laser glasses have focused on modifyingthe glass host structure in order to broaden the bandwidth as well as toimprove cross sections and thermal performance. See, for example, Payneet al. (U.S. Pat. No. 5,663,972) which discloses the use of Nd-dopedphosphor-alumino laser glasses containing MgO. These glasses aredescribed as having broad emission bandwidths. However, the Nd-dopedphosphate glass described therein is difficult to manufacture with highyields. Moreover, there is still a need for a material having evenlarger emission bandwidth.

J. S. Hayden et. al., “Effect of composition on the thermal, mechanicaland optical properties of phosphate laser glasses,” SPIE Vol. 1277(1990), 127-139, describes a study of 41 Nd-doped phosphate laserglasses with respect to certain modifier components. In these glassesthe amount of alkali and alkaline earth metal contents were varied toinvestigate their impact on the thermal, mechanical, optical, and laserproperties of the glasses. In that study, it was determined that theemission cross section remained nearly constant over a wide range ofmodifier compositions.

Other attempts to obtain glasses with broad emission bandwidths haveused a tellurite host material. See, for example, Aitken et al. (U.S.Pat. No. 6,656,859) which describes a tellurite glass doped with erbiumwhich contain 65-97% TeO₂. See also Aitken et al. (U.S. Pat. No.6,194,334) which describes an alkali-tungsten-tellurite glass containing10-90% TeO₂, and Jiang et al. (U.S. Pat. No. 6,859,606) which describesa boro-tellurite glass containing 50-70% TeO₂.

J. H. Yang, et al., “Mixed Former Effects: A Kind of CompositionsAdjusting Method of Er-doped glass for broadband amplification,” Chin.Phys. Lett. 19[10] (2002) 1516-1518, disclose the results of an Er-dopedbismuth-based glass. The glass was found to have a high emission crosssection (σ^(p) _(e)=0.66-0.90 pm²) and large fluorescence FWHM(fluorescence full width at half maximum) (68-95 nm) in comparison toother erbium-doped glasses.

In addition to phosphate and tellurite glasses, silicates, borates,borosilicates, and aluminates have also been used as host glass matrixsystems for lasing ions. Silicate, borate, borosilicates, and aluminateglasses have broader emission bandwidth for Nd lasing ions in comparisonto phosphate glasses.

However, there are disadvantages associated with the use of theseglasses. For example, silicate glasses normally melt at very hightemperatures, unless they contain significant amount of modifiers, suchas alkali metals or alkaline earths metals. Borate glasses, on the otherhand, have low temperature melting characteristics, but they requiresubstantially high concentrations of alkali metals or alkaline earthmetals to be stable in ambient environments. Borosilicate glasses can bedurable at ambient temperatures and are melted at temperaturescomparable to standard commercial glasses, such as the soda-lime glass.However, typical commercial borosilicate glasses contain significantamounts of alkali metals, which promote high borate volatility, similarto phosphate glass, during melting. Aluminate glasses exhibitparticularly broad emission bandwidths and are attractive for shortpulse laser operation. But, these glasses have a very high tendencytowards crystallization.

One general trend in solid state lasers is to make high energy laserswith shorter pulse lengths, which drives the power in the pulse to veryhigh numbers. For example, a 10 k Joule laser with a 10 nsec pulselength is a power of 1 TW (1 TW=10000 J/10 nsec). The trend towards theuse of high energy lasers with shorter pulse lengths is described in“Terrawatt to pettawatt subpicosecond lasers”, M. D. Perry and G.Mourou, Science, Vol 264, 917-924 (1994).

For mode-locked lasers, it is a well-known result, from Fourier'stheorem, that the narrower the pulse width, the larger the gainbandwidth required to generate that pulse; thus said to be transformlimited. For an inhomogeneously broadened line width of a laser medium,if the intensity of pulses follows a Gaussian function, then theresulting mode-locked pulse will have a Gaussian shape with the emissionbandwidth/pulse duration relationship of: Bandwidth X PulseDuration≧0.44. See W. Koechner, Solid State Laser Engineering, bed,Springer Science, 2006 (pg 540). Clearly, to achieve ever shorter pulsedurations it is a requirement to identify glasses with a broad emissionbandwidth.

Thus, an important factor in designing laser systems that utilize shortpulses is to find gain materials with broad emission bandwidths for thelaser transition. The relationship between emission bandwidth and pulselength is: Bandwidth X Pulse Duration ≧0.44. Clearly, the need for evershorter pulse durations necessitates glasses with a broad emissionbandwidth.

Unfortunately, the emission bandwidths achievable in glass hosts aretypically many factors smaller than what is possible in the Ti:Sapphirecrystal. For high peak power lasers using ultra-short pulses (<100femto-second pulses or shorter), the emission bandwidth offered by knownphosphate laser glass is too narrow compared to that required. In orderto overcome this limitation, so-called “mixed” laser glass amplifierapproach is used in order to achieve the total bandwidth necessarybefore the pulse compression. The petawatt laser architecture that is inoperation and producing the highest peak powers available today usesthis methodology. The design of this petawatt laser is shown in, E.Gaul, M. Martinez, J. Blakeney, A. Jochmann, M. Ringuette, D. Hammond,T. Borger, R. Escamilla, S. Douglas, W. Henderson, G. Dyer, A.Erlandson, R. Cross, J. Caird, C. Ebbers, and T. Ditmire, “Demonstrationof a 1.1 petawatt laser based on a hybrid optical parametric chirpedpulse amplification/mixed Nd:glass amplifier,” Appl. Opt. 49, 1676-1681(2010).

In these mixed laser glass designs, phosphate and silicate glasses areused in series to achieve the total bandwidth required. See, forexample, G. R. Hays, et al., “Broad-spectrum neodymium-doped laserglasses for high-energy chirped-pulse amplification,” Appl. Opt. 46[21](2007) 4813-4819, which describes a mixed-glass architecture using anNd-doped tantalum/silicate glass and an Nd-doped aluminate glass.

Though proven, this technology is still insufficient for the futurecompact petawatt and for the future exawatt systems capable of producinghigh energies at shorter pulse durations. New glasses with bandwidthsthat are two and three times larger than what is currently availablefrom laser glasses is needed, if there is to be an alternative toTi:Sapphire for the laser community.

Thus, one aspect of the invention is to provide a solid glass lasermedium having a broader emission bandwidth of rare earth ions used aslasing ions.

According to a further aspect of the invention, there is provided adoped phosphate glass composition for use as a solid laser medium havinga broad emission bandwidth of rare earth ions used as lasing ions. Inparticular, there is provided a phosphate laser glass compositioncontaining Nd₂O₃ and Yb₂O₃ as co-dopants.

Upon further study of the specification and appended claims, furtheraspects and advantages of this invention will become apparent to thoseskilled in the art.

While the prior attempts at broadening the emission bandwidths inglasses have focused on modifying the glass host structure, the presentinvention focuses on the rare-earth dopants, particularly, the energytransfer mechanisms between dopants and the impact of such interactionson laser emission bandwidth. In accordance with the invention, aphosphate glass host is doped with multiple rare-earth dopants,typically Nd₂O₃ in combination with Yb₂O₃. The resulting emissionbandwidths obtained are much broader than what is currently achievablewith a single dopant in glass.

The combination of Nd₂O₃ and Yb₂O₃ has been utilized in other laserglass compositions. De Sousa et. al., “Spectroscopy of Nd³⁺ and Yb³⁺codoped Fluoroindogallate glasses, J. Appl. Phys., Vol. 90, No. 7, 2001,discloses the result of a study on Nd—Nd and Nd—Yb transfer processes incertain lead fluoroindogallate glasses. The glass compositions were30PbF₂-20GaF₃-15InF₃-15ZnF₂-(20-X)CaF₂—XNdF₃ (with X=0.1, 0.5, 1, 2, 4,and 5); 30PbF₂-20GaF₃-15InF₃-15ZnF₂-(20-X)CaF₂—XYbF₃ (with X=0.1, 0.5,1, 2, 3, and 5); and 30PbF₂-20GaF₃-15InF₃-15ZnF₂-(19-X)CaF₂—XYbF₃-1NdF₃(with X=0.1, 0.5, 1, 2, 3, and 5.5).

Rivera-Lòpez et al., “Efficient Nd³⁺→Yb³⁺ Energy Transfer Processes inHigh Photon Energy Phosphate Glasses for 1.0 μm Yb³⁺ Laser,” J. Appl.Phys. 109, 123514 (2011) disclose a study on Nd³⁺→Yb³⁺ energy transferin certain phosphate glasses. The glasses studied have the followingcompositions: 58.5 mol % P₂O₅, 17 mol % K₂O, 14.5 mol % BaO, 9 mol %Al₂O₃, and 1 mol % Nd₂O₃; 58.0 mol % P₂O₅, 17 mol % K₂O, 14.0 mol % BaO,9 mol % Al₂O₃, 1 mol % Nd₂O₃, and 1.0 mol % Yb₂O₃; 57.5 mol % P₂O₅, 17mol % K₂O, 13.5 mol % BaO, 9 mol % Al₂O₃, 1 mol % Nd₂O₃, and 2.0 mol %Yb₂O₃; and 56.5 mol % P₂O₅, 17 mol % K₂O, 12.5 mol % BaO, 9 mol % Al₂O₃,1 mol % Nd₂O₃, and 4.0 mol % Yb₂O₃.

Sontakke et al., “Efficient Non-Resonant Energy Transfer in Nd³⁺—Yb³⁺Codoped Ba—Al metaphosphate Glasses,” J. Opt. Soc. Am. B/Vol. 27, No.12, 2010, disclose a study on the Nd³⁺→Yb³⁺ energy transfer in certainalkali-free barium-alumino-metaphosphate glasses. The glass compositionswere (99-x) [20.95 mol % BaO, 11.72 mol % Al₂O₃, 56.12 mol % P₂O₅, 6.79mol % SiO₂, 3.91B₂O₃, 0.51 mol % Nb₂O₅]+1.0 mol % Nd₂O₃+X mol % Yb₂O₃(X=0, 0.05, 0.1, 0.5, 1.0, 3.0, 6.0, 9.0).

See also E. Yahel et al., “Modeling and Optimization of High-PowerNd³⁺—Yb³⁺ Codoped Fiber Lasers,” J. Lightwave Technology, Vol. 24, No.3, pp. 1601-1609 (March 2006).

Laser glass compositions containing combinations of Nd₂O₃ and Yb₂O₃ arealso described in Miura et al. (U.S. Pat. No. 4,806,138), Myers (U.S.Pat. No. 4,962,067), and Myers (U.S. Pat. No. 7,531,473).

The glasses disclosed herein are suitable for use at powers of more than1000× to 1000000× higher (pettawatt to exawatt levels, or even higher).The disclosed glasses can be used to achieve pulse lengths less than 100fsec and they will have sufficient high gain to get output energiesof >100 kJ. In laser systems, the glasses according to the invention canbe energized by the use of a flashlamp as a pump source. Laser diodepumping is also possible.

In accordance with an aspect of the invention, the phosphate glasscomposition comprises (based on mol %):

P₂O₅ 50.00-70.00 B₂O₃  2.00-10.00 Al₂O₃ 1.00-5.00 SiO₂ 1.00-5.00 Nd₂O₃0.10-5.00 Yb₂O₃  0.10-35.00 La₂O₃  0.00-20.00 Er₂O₃ 0.00-5.00 CeO₂ 0.00-20.00wherein the ratio of Yb₂O₃ to Nd₂O₃ is 1-25 or 0.100-0.333.

In accordance with a further aspect of the invention, the phosphateglass composition is intended for use in a laser that operates Ybwavelengths near 1 μm (1000 nm-1025 nm). In this case, the glasscontains 0.10 to 5.0 mol % Nd₂O₃ and the Yb₂O₃ to Nd₂O₃ ratio is 25-1.

In accordance with a further aspect of the invention, the phosphateglass composition is intended for use in a laser that operates at Ndwavelengths (e.g., 1054 nm). In this case, the glass contains 0.10 to5.0 mol % Nd₂O₃ and the Yb₂O₃ to Nd₂O₃ ratio is 0.100-0.333.

In accordance with a further aspect of the invention, the phosphateglass composition comprises (based on mol %):

P₂O₅ 60-70 B₂O₃  7-10 Al₂O₃ 3-5 SiO₂ 3-5 Nd₂O₃ 0.5-4.0 Yb₂O₃  0.1-25.0La₂O₃ 0.0-15  Er₂O₃ 0.00-5.00 CeO₂ 0.0-15  Cr₂O₃  0.0-1.00 Nb₂O₅ 0.0-1.00 As₂O₃ and/or Sb₂O₃  0.1-1.00

In the above embodiment, As₂O₃ and Sb₂O₃ are used as refining agentsand/or antisolarants. Thus, the total amount of these refiningagents/antisolarants is 0.1-1.0 mol %.

For lasers pumped by flashlamps, CeO₂ and Nb₂O₅ act as antisolarants.

In the above embodiment, the function of Cr₂O₃ may differ depending onthe overall composition of the glass. For example, Cr₂O₃ may function asan auxiliary dopant/sensitizer to increase efficiency.

The general glass composition according to the invention may includealkali and/or alkaline earth metals, for example, MO is 0.00-10.00 mol %wherein MO is the sum of MgO, CaO, SrO, BaO, and ZnO; and M′₂O is0.00-10.00 mol % wherein M′₂O is the sum of Li₂O, Na₂O, K₂O, and Cs₂O.But, in accordance with another aspect of the invention the phosphateglass composition does not contain any alkali or alkaline earth metals.In this case, the absence of alkali and alkaline earth metals provides avery low volatility during melting.

Essentially free of alkali metals means that the glass compositionaccording to the invention contains less than 0.5 mol % of alkali metals(such as Na₂O, Li₂O, and K₂O), especially less than 0.1 mol %.Essentially free of alkaline earth metals means that the phosphate glasscomposition according to the invention contains less than 0.5 mol % ofalkaline earth metals (such as BaO, CaO, and MgO), especially less than0.1 mol %.

With regards to ranges described herein, all ranges include at least thetwo endpoints of the ranges, as well as all units between the twoendpoints. Thus, for example, a range of 1 to 10 is to be understood asexpressly disclosing at least the values of 1, 2, 3, 4, 5, 6, 7, 8, 9,and 10.

In the glass composition according to the invention, P₂O₅ functions asthe source of the primary network former. Thus, according to anotheraspect of the invention, the phosphate glass composition according tothe invention contains 50.00-70.00 mol % of P₂O₅, such as 60.00-70.00mol % of P₂O₅, for example, 60, 61, 62, 63, 64, 65, 66, 67 68, 69, or 70mol % (e.g., 60.00-67.00 mol % of P₂O₅).

In the glass composition according to the invention, B₂O₃ also acts as anetwork former. According to another aspect of the invention, thephosphate glass composition according to the invention contains2.00-10.00 mol % of B₂O₃, such as 2, 3, 4, 5, 6, 7, 8, 9, or 10 mol %.For example, the phosphate glass composition according to the inventionmay contain 3.00-8.00 mol % of B₂O₃ or 3.00-5.00 mol % of B₂O₃, or6.00-8.00 mol % of B₂O₃, or 7.00-10.00 mol % of B₂O₃.

Al₂O₃ can also acts as a network former in the glass composition of theinvention. According to another aspect, the phosphate glass compositionaccording to the invention contains 1.00-5.00 mol % of Al₂O₃, such 1, 2,3, 4, or 5 mol %. For example, the phosphate glass composition accordingto the invention may contain 1.00-4.00 mol % of Al₂O₃ or 1.00-3.00 mol %of Al₂O₃, or 3.00-5.00 mol % of Al₂O₃.

SiO₂ can also acts as a network former in the glass composition of theinvention. According to another aspect, the phosphate glass compositionaccording to the invention contains 1.00-5.00 mol % of SiO₂, such 1, 2,3, 4, or 5 mol %. For example, the phosphate glass composition accordingto the invention may contain 1.00-4.00 mol % of SiO₂ or 1.00-3.00 mol %of SiO₂, or 3.00-5.00 mol % of SiO₂.

Nd₂O₃ and Yb₂O acts as co-dopants and thus both provide the lasingaction of the glass. According to another aspect, the phosphate glasscomposition according to the invention contains 0.10-5.00 mol % ofNd₂O₃, such as 0.2, 0.3, 0.4, 0.5, 0.75, 0.85, 1.0, 1.25, 1.5, 1.75,2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0 mol %. For example, the phosphateglass composition according to the invention may contain 1.00-4.00 mol %of Nd₂O₃ or 1.50-2.50 mol % of Nd₂O₃.

According to another aspect, the phosphate glass composition accordingto the invention contains 0.10-40.00 mol % of Yb₂O₃, such as 0.2, 0.3,0.4, 0.5, 0.75, 0.85, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5, 3.0, 3.5, 4.0,4.5, 5.0, 10.0, 15.0, 20.0, 25.0, 30.0, 35.0, or 40.0 mol %. Forexample, the phosphate glass composition according to the invention maycontain 0.10-20.00 mol % of Yb₂O₃ or 0.10-10.00 mol % of Yb₂O₃ or0.10-1.00 mol % of Yb₂O₃, 0.10-5.00 mol % of Yb₂O₃, or 8.00-10.00 mol %of Yb₂O₃, or 30.00-40.00 mol % of Yb₂O₃.

According to another aspect, the phosphate glass composition accordingto the invention generally contains 0.00-20.00 mol % of La₂O₃, forexample, 0.00-16.00 mol % of La₂O₃ or 0.00-8.00 mol % of La₂O₃ or7.00-16.00 mol % of La₂O₃.

According to another aspect, the sum of the rare earth oxides, Re₂O₃,i.e., the sum of La₂O₃, Nd₂O₃, Yb₂O₃, CeO₂, and Er₂O₃ in the phosphateglass composition according to the invention is preferably 0.2-40 mol %,such as 0.3, 0.4, 0.5, 0.6, 0.75, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0,4.5, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0,17.0, 18.0, 19.0, 20.0, 25.0, 30.0, 35.0, or 40.0 mol %. For example,the phosphate glass composition according to the invention may contain1-25 mol %, or 5-20 mol %, or 15-20 mol % of Re₂O₃.

According to another aspect, the Yb₂O₃ to Nd₂O₃ mole ratio in thephosphate glass composition according to the invention is 0.100-0.333(i.e., Yb:Nd of 1:10 to 1:3), such as 0.10, 0.15, 0.18, 0.2, 0.21, 0.22,0.23, 0.24, 0.25, 0.26, 0.28, 0.29, 0.30, 0.31, 0.32, or 0.33. Forexample, the Yb₂O₃ to Nd₂O₃ ratio in the phosphate glass compositionaccording to the invention may be 0.1-0.2, or 0.15-0.25, or 0.2-0.3.

According to another aspect, the Yb₂O₃ to Nd₂O₃ mole ratio in thephosphate glass composition according to the invention is 1-25 (i.e.,Yb:Nd of 1:1 to 25:1), such as 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5,5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0,18.0, 19.0, 20.0, 21.0, 22.0, 23.0, 24.0, or 25.0. For example, theYb₂O₃ to Nd₂O₃ ratio in the phosphate glass composition according to theinvention may be 1-20, or 2-15, or 2-10, or 5-10.

As mentioned above, the glass composition according to the invention mayinclude alkali metals, M′₂O (sum of Li₂O, Na₂O, K₂O, and Cs₂O), inamounts of 0.00-10.00 mol %, for example, 0.0, 1.0, 2.0, 3.0, 4.0, 5.0,6.0, 7.0, 8.0, 9.0, or 10.0 mol %. The alkali metals can be added to theglass composition to further modify laser and mechanical properties ofthe glass system. See, for example, J. S. Hayden et al., “Effect ofcomposition on the thermal, mechanical and optical properties ofphosphate laser glasses,” SPIE Vol. 1277 (1990), 127-139.

Also, as mentioned above, the glass composition according to theinvention may include alkaline earth metals, MO (sum of MgO, CaO, SrO,BaO, and ZnO), in amounts of 0.00-10.00 mol %, for example, 0.0, 1.0,2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, or 10.0 mol %. The alkalineearth metals can be added to the glass composition to further modifylaser and mechanical properties of the glass system. See, for example,J. S. Hayden et al., “Effect of composition on the thermal, mechanicaland optical properties of phosphate laser glasses,” SPIE Vol. 1277(1990), 127-139.

Overall, the sum of the alkali metals and alkaline earth metals, i.e.,the sum of MO and M′₂O, is 0.00-20.00 mol %, such as 0.0, 1.0, 2.0, 3.0,4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or20 mol %. For example the total amount of alkali metals and alkalineearth metals (sum of MO and M′₂O) in the glass composition can be0.0-15.0 mol %, or 0.0-10.0 mol %, or 0.0-5.0 mol %, or 0.0-3.0 mol %.

In accordance with another aspect of the invention, the phosphatecomposition according to the invention possesses an effective emissionbandwidth (Δλ_(eff)) of at least 35 nm, preferably at least 45 nm,especially at least 100 nm, and in particular at least 105 nm.

In accordance with another aspect of the invention, there is provided alaser system wherein a Yb laser is pumped with a flash lamp. Because Ybhas minimal absorption for flash lamp light, typical lasers that use Ybdoped laser glass for the gain material, use diode technology, which isexpensive for scaling up. With the glass composition according to theinvention, for glasses wherein the Yb:Nd ratio is 1.0 or higher, flashlamp energy can be absorbed by Nd bands and transferred to Yb laserlevel.

Laser properties can be measured in accordance with the Judd-Ofelttheory, the Fuchtbauer-Ladenburg theory, or the McCumber method. Adiscussion of the Judd-Ofelt theory and the Fuchtbauer-Ladenburg theorycan be found in E. Desurvire, Erbium Doped Fiber Amplifiers, John Wileyand Sons (1994). The McCumber method is as discussed in, for example,Miniscalco and Quimby, Optics Letters 16(4) pp 258-266 (1991). See alsoKassab, Journal of Non-Crystalline Solids 348 (2004) 103-107. TheJudd-Ofelt theory and the Fuchtbauer-Ladenburg theory evaluate laserproperties from an emission curve, whereas the McCumber method uses theabsorption curve of the glass. Regarding emission bandwidth, if one hasa measured emission curve (such as collected in a Judd-Ofelt orFuchtbauer-Ladenburg analysis) or a calculated emission curve (from aMcCumber analysis) one can get emission bandwidth in two ways. The firstway is to simply measure the width at half of the maximum value (calledemission bandwidth full width half maximum or Δλ_(FWHM)).

An emission curve for Yb will exhibit a narrow feature at ˜980 nm. Ifthis feature is prominent, the Δλ_(FWHM) value will only reflect thewidth of this one feature and the rest of the curve will not contribute.As a result the Δλ_(FWHM) value is not always a reliable indicator ofthe emission bandwidth for Yb.

The second method divides every point on the emission curve by the totalarea under the curve. The result, called a linewidth function, will havea peak value that is defined as the inverse of the effective bandwidth,Δλ_(eff). By this method the entire emission curve always contributed tothe emission bandwidth result. It is this value used herein in theanalysis as the best indicator of emission bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further details, such as features and attendantadvantages, of the invention are explained in more detail below on thebasis of the exemplary embodiments which are diagrammatically depictedin the drawings, and wherein:

FIG. 1 graphically illustrates the experimental emission spectraobtained at different pump energy settings for a Ce—Yb—Nd co-doped glasssystem;

FIG. 2 graphically illustrates experimental emission spectra obtained atdifferent pump energy settings for the La—Yb—Nd co-doped glass system;

FIG. 3 graphically illustrates emission bandwidth changes as a functionof dopant concentrations; and

FIG. 4 illustrates a comparison of emission curves of the ultra-broadbandwidth laser glasses according to the invention which contain rareearth mixture with the prior art commercial phosphate glass, LG770.

EXAMPLES

Table 1 lists compositions in accordance with the invention. Inaddition, Table 4 lists comparison example glass composition wherein theglasses do not contain a Nd—Yb co-doped system. Table 2 lists physicaland optical properties of the glasses of Table 1. Table 3 lists theEmission Bandwidth for the glasses of Table 1. Table 5 lists physicaland optical properties of the glasses of Table 4. Table 6 lists theEmission Bandwidth for the glasses of Table 4.

All of the glasses were made using laser grade ingredients and meltedunder a dry oxygen environment with the help of stirring using aplatinum stirrer for better homogeneity. All of the glasses were castinto a mold and this was appropriately annealed in order to remove thestress as the liquid cools to the amorphous state. The resulting glassslab was shaped into forms required for use with the instruments thatprovide various properties for glasses.

The results of these property measurements and calculations are detailedin Table 2 for the glasses included in this invention and in Table 5 fora comparison example.

The fluorescence emission spectrum was obtained by exciting the dopantions with an Argon laser where the pump wavelength is set at 514 nm. Theresulting emission spectrum was collected using a 0.3 m imaging triplegrating spectrometer and a 3 mm InGaAs near infrared detector.Step-scans at 0.1 nm intervals were obtained with a 600 grooves/mgrating in the spectrometer. A sample of each glass was used to measurethe emission spectrum, from which the effective emission bandwidth(Δλ_(eff)) was determined according to Equation (1):

$\begin{matrix}{{\Delta\lambda}_{eff} = {\frac{\int_{800}^{1200}{{I(\lambda)}{\lambda}}}{I_{\max}}.}} & (1)\end{matrix}$

The integrated area of the spectrum was made between 800 and 1200 nm.Each curve that was collected is post-processed using a FFT smoothingfilter of appropriate length. The smoothed spectra were used forcalculations in order to reduce the noise level in the data sets. Seefor example FIG. 1 and FIG. 2 which show the data after smoothing.

In FIG. 1 and FIG. 2 are emission curves for two examples, CYN-1 andLYN-4. In each case three emission peaks are noted in the emissionspectra. The peaks at nominally 980 nm and 1000 nm are assigned to Yb₂O₃and the peak at nominally 1060 nm is assigned to Nd₂O₃. It should benoted that ytterbium has self-absorption in the region near 980 nm, sonot all of the emission in the example glasses can be effectivelyutilized in an actual laser system.

In FIG. 1, nearly 110 nm of bandwidth is calculated from the measuredspectra for the Ce—Yb—Nd co-doped glass. In FIG. 2, nearly 105 nm ofbandwidth is calculated for the La—Yb—Nd co-doped glass.

In Table 3 it is shown that LYN-2 glass with a Nd₂O₃/Yb₂O₃ ratio of ˜2.3produces the smallest effective emission bandwidth compared to the otherexample glasses with the Nd₂O₃/Yb₂O₃ that is higher or lower than LYN-2.

FIG. 3 illustrates how the emission spectrum, and consequently theeffective emission bandwidth, can be tuned and extended with the optimalselection of doping levels of both Nd₂O₃ and Yb₂O₃. The key tomaximizing the emission bandwidth is to adjust the doping concentrationsso that the three emission peaks are produced at nominally the sameintensity. As can be seen from the Figure, selection of the optimaldoping ratio between Nd₂O₃ and Yb₂O₃ can increase the emission bandwidthfrom about 45 nm to over 100 nm

FIG. 4 illustrates a comparison of emission curves of the ultra-broadbandwidth laser glasses according to the invention which contain rareearth mixture with the prior art phosphate glass, LG770, which containsP₂O₅, Al₂O₃, K₂₀, MgO, and Nd₂O₃ (See U.S. Pat. No. 5,526,639). As canbe seen, the glass composition of the invention, CYN-1, has asignificantly broader bandwidth than that of LG770.

TABLE 1 Glass Compositions (mol %) of New Ultra-broad Bandwidth LaserGlasses Containing Rare Earth Mixtures Mol % Example Oxide YN-1 LYN-1LYN-2 LYN-3 LYN-4 CYN-1 SiO₂ 1.364 4.001 4.001 4.001 4.001 4.001 B₂O₃3.158 7.992 7.992 7.992 7.992 7.992 Al₂O₃ 2.315 4.001 4.001 4.001 4.0014.001 P₂O₅ 53.133 65.955 65.955 65.955 65.955 65.955 CeO₂ 15.352 Nd₂O₃2.005 2.100 1.965 1.900 1.950 1.673 Yb₂O₃ 38.025 7.975 0.850 0.200 0.4200.358 La₂O₃ 7.975 15.235 15.235 15.235 Nd₂O₃/Yb₂O₃ 0.053 0.263 2.3129.500 4.643 4.673 Yb₂O₃/Nd₂O₃ 18.965 3.798 0.433 0.105 0.215 0.214

TABLE 2 Properties of New Ultra-broad Bandwidth Laser Glasses ContainingRare Earth Mixtures Example Property YN-1 LYN-1 LYN-2 LYN-3 LYN-4 CYN-1Refractive Index, nd 1.54794 1.56212 1.58040 1.58438 1.58312 1.56140Abbe Number, Vd 62.77 62.62 61.57 61.39 61.21 59.20 Refractive Index at1054 nm, n₁₀₅₄ 1.539 1.553 1.571 Nonlinear Index, n₂ [esu] 1.31 1.373.96 Absorption at 3000 nm [cm⁻¹] 0.190 1.11 0.56 0.60 Absorption at3333 nm [cm⁻¹] 0.589 1.48 0.98 1.04 Density [gm/cm³] 3.403 3.260 3.2073.204 3.200 2.997 Thermal Expansion from 20- 4.79 5.82 6.61 300° C.[ppm/K] Transformation Point, 769.3 706.3 NA Tg(DTA) [° C.]Transformation Point, Tg NA NA 651.0 (Dilatometer) [° C.] ThermalConductivity at 0.5106 0.51 0.57 25° C., K_(25° C.) [W/mK] ThermalConductivity at 0.629 0.63 0.61 90° C., K_(90° C.) [W/mK] Poisson'sRatio 0.23 0.24 0.25 Young's Modulus, E [GPa] 66.40 64.0 64.2 FractureToughness, K_(1C) 0.825 0.78 0.86 [MPa/m^(1.5)] Hardness, HK 412.4 380410

TABLE 3 Emission Bandwidth of New Ultra-broad Bandwidth Laser GlassesContaining Rare Earth Mixtures Example Property YN-1 LYN-1 LYN-2 LYN-3LYN-4 CYN-1 Nd₂O₃/Yb₂O₃ 0.053 0.263 2.312 9.500 4.643 4.673 RatioYb₂O₃/Nd₂O₃ 18.965 3.798 0.433 0.105 0.215 0.214 Ratio Emission 129.790.3 45.7 83.3 105.1 108.8 Bandwidth [nm]

TABLE 4 Glass Compositions (mol %) of Prior Art Glasses withoutIncorporation of Multiple Rare Earth Mixtures Example Mol % Oxide Y-1N-1 SiO₂ 4.001 4.001 B₂O₃ 7.992 7.992 Al₂O₃ 4.001 4.001 P₂O₅ 65.95565.955 CeO₂ Nd₂O₃ 1.000 Yb₂O₃ 18.051 La₂O₃ 17.051

TABLE 5 Properties of Prior At Glasses without Incorporation of MultipleRare Earth Mixtures Example Property Y-1 N-1 Refractive Index, nd1.55087 1.58720 Abbe Number, Vd 62.76 61.26 Refractive Index at 1054 nm,n₁₀₅₄ 1.542 1.577 Nonlinear Index, n₂ [esu] 1.32 1.52 Absorption at 3000nm [cm⁻¹] 0.744 0.53 Absorption at 3333 nm [cm⁻¹] 1.273 0.95 Density[gm/cm³] 3.470 3.232 Thermal Expansion from 20-300° C. [ppm/K] 47.7 68.7Transformation Point, Tg (DTA) [° C.] 758.7 NA Transformation Point, Tg(Dilatometer) [° C.] too high 650.0 Thermal Conductivity at 25° C.,K_(25° C.) [W/mK] 0.5107 0.52 Thermal Conductivity at 90° C., K_(90° C.)[W/mK] 0.6253 0.66 Poisson's Ratio 0.23 0.27 Young's Modulus, E [GPa]67.73 64.1 Fracture Toughness, K_(1C) [MPa/m^(1.5)] 0.825 0.78 Hardness,HK 408.9 420

TABLE 6 Emission Bandwidth of Prior Art Glasses without Incorporation ofMultiple Rare Earth Mixtures Example Property Y-1 N-1 Emission Bandwidth[nm] 76.4 33.45

The entire disclosure[s] of all applications, patents and publications,cited herein, are incorporated by reference herein.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. An Nd-doped and Yb-doped phosphate glass composition comprising(based on mol %): P₂O₅ 50.00-70.00 B₂O₃  2.00-10.00 Al₂O₃ 1.00-5.00 SiO₂1.00-5.00 Nd₂O₃ 0.10-5.00 Yb₂O₃  0.10-35.00 La₂O₃  0.00-20.00 Er₂O₃0.00-5.00 CeO₂  0.00-20.00

wherein the mole ratio of Yb₂O₃ to Nd₂O₃ is 1-25 or 0.100-0.333.
 2. AnNd-doped and Yb-doped phosphate glass composition according to claim 1,wherein the Yb₂O₃ to Nd₂O₃ mole ratio is 1-25.
 3. An Nd-doped andYb-doped phosphate glass composition according to claim 2, wherein theYb₂O₃ to Nd₂O₃ mole ratio is 1-20.
 4. An Nd-doped and Yb-doped phosphateglass composition according to claim 1, wherein the Yb₂O₃ to Nd₂O₃ ratiois 0.100-0.333.
 5. An Nd-doped and Yb-doped phosphate glass compositionaccording to claim 4, wherein the Yb₂O₃ to Nd₂O₃ mole ratio is or0.2-0.3.
 6. An Nd-doped and Yb-doped phosphate glass compositionaccording to claim 1, wherein said glass composition contains 0.00-10.00mol % MO and 0.00-10.00 mol % M′₂O, wherein MO is the sum of MgO, CaO,SrO, BaO, and ZnO, and M′₂O is the sum of Li₂O, Na₂O, K₂O, and Cs₂O. 7.An Nd-doped and Yb-doped phosphate glass composition according to claim1, wherein said glass composition contains less than 0.5 mol % of alkalimetals.
 8. An Nd-doped and Yb-doped phosphate glass compositionaccording to claim 1, wherein said glass composition contains less than0.5 mol % of alkaline earth metals.
 9. An Nd-doped and Yb-dopedphosphate glass composition according to claim 1, wherein said glasscomposition contains 60.00-70.00 mol % of P₂O₅.
 10. An Nd-doped andYb-doped phosphate glass composition according to claim 1, wherein saidglass composition contains 3.00-8.00 mol % of B₂O₃.
 11. An Nd-doped andYb-doped phosphate glass composition according to claim 1, wherein saidglass composition contains 1.00-4.00 mol % of Al₂O₃.
 12. An Nd-doped andYb-doped phosphate glass composition according to claim 1, wherein saidglass composition contains 1.00-4.00 mol % of SiO₂.
 13. An Nd-doped andYb-doped phosphate glass composition according to claim 1, wherein saidglass composition contains 1.00-4.00 mol % of Nd₂O₃.
 14. An Nd-doped andYb-doped phosphate glass composition according to claim 1, wherein saidglass composition contains 0.10-20.00 mol % of Yb₂O₃.
 15. An Nd-dopedand Yb-doped phosphate glass composition according to claim 1, whereinsaid glass composition contains 0.00-16.00 mol % of La₂O₃.
 16. AnNd-doped and Yb-doped phosphate glass composition according to claim 1,wherein the sum of La₂O₃, Nd₂O₃, Yb₂O₃, CeO₂, and Er₂O₃ in the phosphateglass composition is 1-25 mol %.
 17. An Nd-doped and Yb-doped phosphateglass composition according to claim 1, wherein the sum of MO and M′₂O,is 0.00-15.00 mol %.
 18. An Nd-doped and Yb-doped phosphate glasscomposition according to claim 1, wherein the phosphate composition hasan effective emission bandwidth (Δλ_(eff)) of at least 35 nm.
 19. AnNd-doped and Yb-doped phosphate glass composition according to claim 1,wherein said glass composition comprises (based on mol %): P₂O₅ 60-70B₂O₃  7-10 Al₂O₃ 3-5 SiO₂ 3-5 Nd₂O₃ 0.5-4.0 Yb₂O₃  0.1-25.0 La₂O₃0.0-15  Er₂O₃ 0.00-5.00 CeO₂ 0.0-15  Cr₂O₃  0.0-1.00 Nb₂O₅  0.0-1.00As₂O₃ and/or Sb₂O₃   0.1-1.00.


20. A solid state Yb laser system comprising a Yb-doped phosphate glasscomposition according to claim 1, as the solid gain medium and at leastone flash lamp as the pumping source, wherein the Yb:Nd ratio of saidglass composition is 1.0 or higher.
 21. In a solid state laser systemcomprising a solid gain medium and a pumping source, the improvementwherein said solid gain medium is a glass having a composition accordingto claim
 1. 22. A laser system according to claim 21, wherein the poweroutput of system is at least a pettawatt per pulse or greater.
 23. Amethod for generating a laser beam pulse comprising flash lamp pumpingor diode pumping a glass composition according to claim
 1. 24. A methodfor generating a laser beam pulse comprising flash lamp pumping a lasersystem according to claim 21.